F# Sequence Workflows yield and yield!
Example
In sequence workflows, yield adds a single item into the sequence being built. (In monadic terminology, it is return.)
> seq { yield 1; yield 2; yield 3 }
val it: seq<int> = seq [1; 2; 3]
> let homogenousTup2ToSeq (a, b) = seq { yield a; yield b }
> tup2Seq ("foo", "bar")
val homogenousTup2ToSeq: 'a * 'a -> seq<'a>
val it: seq<string> = seq ["foo"; "bar"]
yield! (pronounced yield bang) inserts all the items of another sequence into this sequence being built. Or, in other words, it appends a sequence. (In relation to monads, it is bind.)
> seq { yield 1; yield! [10;11;12]; yield 20 }
val it: seq<int> = seq [1; 10; 11; 12; 20]
// Creates a sequence containing the items of seq1 and seq2 in order
> let concat seq1 seq2 = seq { yield! seq1; yield! seq2 }
> concat ['a'..'c'] ['x'..'z']
val concat: seq<'a> -> seq<'a> -> seq<'a>
val it: seq<int> = seq ['a'; 'b'; 'c'; 'x'; 'y'; 'z']
Sequences created by sequence workflows are also lazy, meaning that items of the sequence don't actually get evaluated until they're needed. A few ways to force items include calling Seq.take (pulls the first n items into a sequence), Seq.iter (applies a function to each item for executing side effects), or Seq.toList (converts a sequence to a list). Combining this with recursion is where yield! really starts to shine.
> let rec numbersFrom n = seq { yield n; yield! numbersFrom (n + 1) }
> let naturals = numbersFrom 0
val numbersFrom: int -> seq<int>
val naturals: seq<int> = seq [0; 1; 2; ...]
// Just like Seq.map: applies a mapping function to each item in a sequence to build a new sequence
> let rec map f seq1 =
if Seq.isEmpty seq1 then Seq.empty
else seq { yield f (Seq.head seq1); yield! map f (Seq.tail seq1) }
> map (fun x -> x * x) [1..10]
val map: ('a -> 'b) -> seq<'a> -> 'b
val it: seq<int> = seq [1; 4; 9; 16; 25; 36; 49; 64; 81; 100]
